Semiconductor devices play a significant role in solving the energy challenges. Specifically, nitride power transistors have great potential in the application of advanced transportation systems, more robust energy delivery networks and many new revolutionary approaches to high-efficiency electricity generation and conversion. Those systems rely on very efficient converters to step-up or step-down electric voltages. Nowadays, these devices are mainly made of silicon (Si). However, the limited breakdown voltage and frequency response of Si, and its higher resistance make the commercial devices and circuits currently available very bulky, heavy and inappropriate for future power applications. As an alternative, gallium nitride (GaN) devices have achieved record combination of high-voltages, high frequency response and low on-resistances for power applications.
Currently, GaN power devices, such as the GaN-based high electron mobility transistors (HEMTs), are regarded as one of the most promising candidates for high-power, high-voltage and high frequency applications. GaN HEMTs, with a two-dimensional-electron gas (2DEG) channel, have achieved up to 10 times higher power density of GaAs HEMTs with much larger breakdown voltage (VB) and current density, as well as a high cut-off frequency of over 400 GHz. State-of-the-art power levels have been demonstrated on SiC substrates with total output powers of 800 W at 2.9 GHz and over 500 W at 3.5 GHz.
There are many circuit applications in digital, microwave and power electronics requiring monolithic or single-device integration of n-channel, such as the 2DEG channel, and p-channel, such as the 2DHG channel. The structure of the nitride materials, such as GaN, can enable a high-speed and high-current n-channel. However, performance of the p-channel of these materials is much inferior, which inhibited the realization of the circuit applications. For example, the effective hole mass in GaN is much higher than the electron mass. The state-of-the-art GaN-based 2DHG p-type channel can only have a mobility of 10-40 cm2/Vs and maximum output current of ˜30-40 mA/mm. In comparison, the n-type 2DEG channel has a mobility of over 2000 cm2/Vs and maximum output current of over 1-2 A/mm) To that end, current efforts to create 2DHG in GaN-based heterostructrues to enable p-channel GaN-based HFETs are largely unsuccessful.